White House spokesman Sean Spicer on Wednesday offered a fresh apology for minimising the atrocities of Adolf Hitler in his comments on Syria, amid a wave of indignation at home and abroad.

"I made a mistake, there is no other way to say it," Mr Spicer told a forum at the Newseum in Washington, adding that the damage was especially unfortunate because "it's a very holy week for the Christian people and the Jewish people".

Mr Spicer said his gaffe was "painful to myself" and was "disappointing because I think I've let the president down".

During a White House briefing on Tuesday, the spokesman for President Donald Trump sought to highlight criticism of Syrian strongman Bashar al-Assad, for his apparent use of the deadly nerve agent sarin against civilians.

"You had someone as despicable as Hitler who didn't even sink to using chemical weapons," Mr Spicer told reporters, momentarily overlooking the Holocaust and prompting outraged calls for his resignation.

A few hours later, a contrite Mr Spicer appeared on US television expressing regret for the comments.

Despite the apologies, Mr Spicer's remarks provoked outrage around the globe from those who claimed he showed insensitivity to victims of the Holocaust.

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German Chancellor Angela Merkel's spokesman said contemporary comparisons with Nazi atrocities were ill-advised.

"Any comparison of current situations with the crimes of National Socialism leads to nothing good," the spokesman, Steffen Seibert, told reporters.

In Israel, the Yad Vashem memorial center urged Mr Spicer to visit its website and expressed "deep concern regarding the inaccurate and insensitive use of terms related to the Holocaust" by the press secretary.

"His statements imply a profound lack of knowledge of events of the Second World War, including the Holocaust. Moreover, they are liable to strengthen the hands of those whose goal is to distort history," it said.

In his first attempts to clarify his remarks Tuesday, Mr Spicer appeared to compound his error, saying: Hitler "brought them into the Holocaust centres, I understand that. But I'm saying in the way that Assad used them, where he went into towns, dropped them down, to innocent -- into the middle of towns".

Mr Spicer -- the most public face of Trump's administration after the president himself -- has been a frequent target of ire and satirists for his angry denunciations of press coverage and sometimes loose grasp of the facts.

Mr Spicer's comments, which came during the Jewish festival of Passover, sparked calls from anti-defamation groups and opposition Democrats for his ouster.

"Sean Spicer must be fired, and the president must immediately disavow his spokesman's statements," top congressional Democrat Nancy Pelosi said.

"Either he is speaking for the president, or the president should have known better than to hire him."

Steven Goldstein, head of the Anne Frank Center, described Mr Spicer's comments as an "evil slur" and said he now "lacks the integrity to serve".

"On Passover, no less, Sean Spicer has engaged in Holocaust denial, the most offensive form of fake news, by denying Hitler gassed millions of Jews to death," Goldstein said.

The American Jewish Committee also denounced the comments, saying it was "astonished".

"What did the Nazis use to exterminate millions of Jews if not chemicals in their death camps?" AJC chief David Harris said.